1. Field of the Invention
The invention relates to newly identified mammalian chemosensory G protein-coupled receptors, to family of such receptors, and to the genes and cDNA encoding said receptors. More particularly, the invention relates to newly identified mammalian chemosensory G protein-coupled receptors active in taste signaling, to a family of such receptors, to the genes and cDNA encoding said receptors, and to methods of using such receptors, genes, and cDNA in the analysis and discovery of taste modulators. The invention provides in particular a DNA sequence encoding a novel human taste receptor identified infra as T1R2 and the corresponding receptor polypeptide.
2. Description of the Related Art
The taste system provides sensory information about the chemical composition of the external world. Mammals are believed to have at least five basic taste modalities: sweet, bitter, sour, salty, and umami. See, e.g., Kawamura et al., Introduction to Umami: A Basic Taste (1987); Kinnamon et al., Ann. Rev. Physiol., 54:715-31 (1992); Lindemann, Physiol. Rev., 76:718-66 (1996); Stewart et al., Am. J. Physiol., 272:1-26 (1997). Each taste modality is thought to be mediated by a distinct protein receptor or receptors that are expressed in taste receptor cells on the surface of the tongue (Lindemann, Physol. Rev. 76:718-716 (1996)). The taste receptors that recognize bitter, sweet, and umami taste stimuli belong to the G-protein-coupled receptor (GPCR) superfamily (Hoon et al., Cell 96:451 (1999); Adler et al., Cell 100:693 (2000)). (Other taste modalities are believed to be mediated by ion channels.)
G protein-coupled receptors mediate many other physiological functions, such as endocrine function, exocrine function, heart rate, lipolysis, and carbohydrate metabolism. The biochemical analysis and molecular cloning of a number of such receptors has revealed many basic principles regarding the function of these receptors. For example, U.S. Pat. No. 5,691,188 describes how upon a ligand binding to a GPCR, the receptor undergoes a conformational change leading to activation of a heterotrimeric G protein by promoting the displacement of bound GDP by GTP on the surface of the Ga subunit and subsequent dissociation of the Ga subunit from the Gb and Gg subunits. The free Ga subunits and Gbg complexes activate downstream elements of a variety of signal transduction pathways.
Complete or partial sequences of numerous human and other eukaryotic chemosensory receptors are currently known. See, e.g., Pilpel, Y. and Lancet, D., Protein Science, 8:969-977 (1999); Mombaerts, P., Annu. Rev. Neurosci., 22:487-50 (1999). See also, EP0867508A2, U.S. Pat. No. 5,874,243, WO 92/17585, WO 95/18140, WO 97/17444, WO 99/67282. Because of the complexity of ligand-receptor interactions, and more particularly taste stimulus-receptor interactions, information about ligand-receptor recognition is lacking.
The identification and characterization of the GPCRs that function as sweet and umami taste receptors could allow for new methods of discovery of new taste stimuli. For example, the availability of receptors could permit the screening for receptor modulators. Such compounds would modulate taste and could be useful in the food industry to improve the taste of a variety of consumer products; e.g., improving the palatability of low-calorie beverages through the development of new artificial sweeteners.
In part, the present invention addresses the need for better understanding of the interactions between chemosensory receptors and chemical stimuli. The present invention also provides, among other things, novel chemosensory receptors, and methods for utilizing such receptors, and the genes a cDNAs encoding such receptors, to identify molecules that can be used to modulate chemosensory transduction, such as taste sensation.